An unmanned NASA probe made history 117 million miles from Earth on Saturday when it arrived at the huge asteroid Vesta, making it the first spacecraft ever to orbit an object in the solar system's asteroid belt.

The Dawn spacecraft entered orbit around Vesta after a four-year chase and will spend about a year studying the huge space rock before moving on to visit another asteroid called Ceres.

Science editor Alan Boyle's blog: "Astronaut Abby" is at the controls of a social-media machine that is launching the 15-year-old from Minnesota to Kazakhstan this month for the liftoff of the International Space Station's next crew.

Vesta is a huge asteroid about as wide as U.S. state of Arizona, and is also the brightest asteroid in the solar system. It is located in the asteroid belt, a band of rocky objects that encircles the sun between the orbits of Mars and Jupiter. [Photos: Asteroid Vesta and NASA's Dawn Probe]

"Today, we celebrate an incredible exploration milestone as a spacecraft enters orbit around an object in the main asteroid belt for the first time," NASA Administrator Charles Bolden said in a statement. "Dawn's study of the asteroid Vesta marks a major scientific accomplishment and also points the way to the future destinations where people will travel in the coming years. President Obama has directed NASA to send astronauts to an asteroid by 2025, and Dawn is gathering crucial data that will inform that mission."

Biggest asteroids up close
NASA launched the $466 million Dawn mission in 2007 to explore the largest asteroids in the asteroid belt. Vesta is 330 miles (530 kilometers) wide, large enough that some astronomers consider it to be a protoplanet. Astronomers do not understand why the asteroid is so bright and hope Dawn will answer that and other mysteries of Vesta.

After studying Vesta in unprecedented detail, the Dawn probe is expected fire up its ion propulsion system to leave orbit and head to the largest asteroid in the solar system, the dwarf planet Ceres. Ceres is about 590 miles (950 kilometers) wide. Dawn will arrive at this target in 2015, NASA officials said. [7 Strangest Asteroids in the Solar System]

Since its launch, Dawn has covered more than 1.7 billion miles (2.7 billion kilometers).

Late Saturday, the spacecraft beamed a message to Earth to alert its controllers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., that it had begun orbiting Vesta. But the exact time of the probe's asteroid arrival is not yet known, NASA officials said. Mission managers initially estimated that the time of arrival would be at about 10 p.m. PT Friday (1 a.m. ET Saturday).

"The time of Dawn's capture depended on Vesta's mass and gravity, which only has been estimated until now," mission managers said in a statement. "The asteroid's mass determines the strength of its gravitational pull. "

The more massive Vesta is, the stronger its gravity will be, meaning Dawn would have been pulled into orbit earlier. If the asteroid is less massive, the gravitational pull would be weaker, and Dawn would have taken longer to reach orbit.

Missions to asteroid
But arrival time aside, the Dawn probe is most assuredly blazing a new trail in space, NASA officials said.

While past missions by NASA and other space agencies have sent spacecraft to visit asteroids, none of those targets were in the main asteroid belt.

NASA

An artist's conception shows NASA's Dawn spacecraft, propelled by an ion drive, and its two targets: the asteroid Vesta on the left, and the dwarf planet Ceres on the right.

In 2000, a NASA probe called Near-Earth Asteroid Rendezvous Shoemaker (or NEAR Shoemaker) went into orbit around the asteroid Eros and landed on the space rock at the end of its mission. Japan's Hayabusa mission sent a probe that collected samples from the asteroid Itokawa and brought them back to Earth last year. The orbits of Eros as well as Itokawa range outside the main belt.

Dawn's mission was first approved by NASA in 2001, a year after the NEAR Shoemaker arrival at Eros. But budget issues prompted NASA to cancel the mission in March 2006, which sparked an outcry from researchers. NASA reinstated the mission just weeks after its cancellation.

NASA is now planning a new asteroid mission called Osiris-Rex, which is set to launch a spacecraft to a near-Earth asteroid in 2016 and collect samples from the space rock in 2020. That mission is expected to return any samples it collects to Earth in 2023.

You can follow Space.com Managing Editor Tariq Malik on Twitter: @tariqjmalik. Follow Space.com for the latest in space science and exploration news on Twitter @Spacedotcom and on Facebook.

Southern stargazing

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(Y. Beletsky / ESO)
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A balloon's view

Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich.
(Kyle Moroney / AP)
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Spacemen at work

Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment.
(NASA)
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Special delivery

The International Space Station's Canadian-built robotic arm moves toward Orbital Sciences Corp.'s Cygnus autonomous cargo craft as it approaches the station for a Jan. 12 delivery. The mountains below are the southwestern Alps.
(NASA)
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Accidental art

A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288.
(NASA / ESA)
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Supersonic test flight

A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

Red lagoon

The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes.
(ESO/VPHAS team)
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Fire on the mountain

This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people.
(NASA via AP)
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Where stars are born

An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues.
(NASA / JPL-Caltech)
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A long, long time ago...

This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang.
(NASA / NASA via AFP - Getty Images)
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Frosty halo

Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.